PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: No more than 1 type of prior chemotherapy for prostate cancer No prior carboplatin or cisplatin for prostate cancer Endocrine therapy: See Disease Characteristics At least 4 weeks since prior flutamide or nilutamide (6 weeks for bicalutamide) Concurrent primary hormonal therapy allowed (e.g., gonadotropin-releasing hormone agonist or antagonist) Radiotherapy: At least 1 month since prior radiotherapy At least 2 months since prior strontium chloride Sr 89 or samarium Sm 153 lexidronam pentasodium Surgery: Concurrent orchiectomy allowed Other: At least 7 days since prior thiazide diuretic At least 30 days since prior investigational therapy No prior calcitriol for prostate cancer No concurrent magnesium-containing antacids, bile-resin binders, or calcium supplements